Methods and means for measurement of the water-oil interface within a reservoir using an x-ray source

ABSTRACT

An x-ray-based reservoir evaluation tool for measurement variations in formation density anticipated at the water-oil interface of a reservoir is provided, the tool including at least: an internal length comprising a sonde section, wherein said sonde section further comprises an x-ray source; radiation measuring detectors; sonde-dependent electronics; and a plurality of tool logic electronics and PSUs. A method of using an x-ray based reservoir evaluation tool for measuring variations in formation density anticipated at the water-oil interface of a reservoir is also provided, the method including at least the following steps: using x-rays to illuminate the formation surrounding the cased borehole; uses detectors to directly measure the density of the formation; using detectors to directly measure the effects on the measurement from tool stand-off or production liner attenuation; and employing techniques for compensating for the production liner and liner-annular region when computing the saturated formation density within the production interval.

TECHNICAL FIELD

The present invention relates generally to management of hydrocarbonproducing wells, and in particular though non-limiting embodiment tomethods and means for directly determining the location of an oil-waterinterface for predicting reservoir fluid movement, and reservoir fluidratios in a producing well through the use of x-ray measurementtechniques.

BACKGROUND

Well or borehole logging is the practice of making an accurate record,known as a well log, of the geologic formations through which a boreholecreates a path or conduit. Well logging activities are performed duringall phases of an oil and gas well's development, e.g., drilling andevaluation, completion, production and abandonment.

The oil and gas industry logs rock and fluid properties to findhydrocarbon-bearing strata in the formations intersected by a borehole.The logging procedure consists of lowering a tool on the end of awireline into the well to measure the properties of the formation. Aninterpretation of these measurements is then made to locate and quantifypotential zones containing hydrocarbons and at which depths these zonesexist.

Logging is usually performed as the logging tools are pulled out of thehole. This data is recorded in real-time via a data connection to thesurface logging unit or using a memory unit aboard the tool to createeither a printed record or electronic presentation called a well logwhich is then provided to the client. Well logging is performed atvarious intervals during the drilling of the well and when the totaldepth is drilled.

In a petroleum reservoir, oil is produced through a well under pressureof gas, water, or compaction. Water may be naturally present in thereservoir displacing the oil to urge it out through the well bore.Often, water is injected into the reservoir from an injection borelocated near the production bore. As oil is extracted from the well, thewater moves through the porous medium of the formation closer to thewell and the oil-water interface changes shape. If the location of theoil-water interface is not monitored during production, it is possiblethat the well will produce a mixture of oil and water. In some cases, itis possible for the well to produce more water than oil.

Well logs are a primary source of information used to map thedistribution of fluids in hydrocarbon reservoirs. Because of the highelectrical resistivity of hydrocarbons compared to formation water, openhole well logs of resistivity are typically used to infer watersaturation, the percentage of pore volume occupied by water. As wellsare typically cased with conductive steel pipe after drilling, it is notusually possible to take resistivity measurements through the casing. Ifa non-conductive casing is used, cross-hole tomography techniques can beused to map the distribution of electrical resistivity in the reservoirvolume.

Another possible technique would be to measure the bulk electron densityof the reservoir formation directly. This could be achieved by usingstrong gamma emitters (such as Cesium-137). Compton scattering withinthe irradiated reservoir would result in returning gamma rays. However,the amount of ¹³⁷Cs permitted in oilfield operations is limited byregulation to 1.5 Ci. As a result, the fluence of the source would notbe viable at normal logging speeds (1,800 ft/hr.) to produce thestatistics (of returning photons) necessary to make the measurementaccurate enough to determine the differences in the saturation of waterand oil within the formation.

The number of Compton scattering collisions is related directly to thenumber of the electrons per unit volume, or electron density, within theformation. Consequently, the electron density determines the response ofthe density tool.

Wellbore logging operations within the oil and gas industry currentlyuse radioactive isotopes for the purpose of a ready supply of gamma-rayswhich are used in the evaluation of the geological formationssurrounding a borehole.

The use of radioactive isotopes within oilfield operations such as theproduction, logistics, handling, operational use and disposal of suchsources is controlled by regulation. The transport of such isotopesacross geographical and political borders is heavily regulated andcontrolled, due to the risk associated with the potential to cause harmto humans, either accidentally or intentionally, through the directdispersal of the radioactive materials across a populated region orindirectly via introduction into the food chain.

However, the use of such isotopes is tolerated as there has been noviable replacement for the technology until recently. The ability toreplace radioactive isotopes with devices and methods which to notutilize radioactive materials boasts a number of key advantages whenconsidering all aspects of the operational cycle of a wellbore log, fromcommercial to health, safety and the environment.

The use of ¹³⁷Cs within oilfield operations is controlled throughgovernmental regulation, export treaties and embargos. Generally, suchisotopes are produced in a nuclear reactor in the country in which theyare intended to be used. The transport of such isotopes acrossgeographical and political borders is heavily regulated and controlled,due to the risk associated with the potential to cause harm to humans,either accidentally or intentionally, through the direct dispersal ofthe radioactive materials across a populated region or indirectly viaintroduction into the food chain.

After entering the body, ¹³⁷Cs is generally uniformly distributedthroughout the body, with higher concentrations manifesting in muscletissues and lower concentrations in bones. The biological half-life of¹³⁷Cs is about 70 days. Experiments on canines showed that a single doseof 0.0038 Curie per kilogram is lethal within three weeks. Densitylogging operations in oilfield typically use 1.1 Curie of ¹³⁷Cs whichequates of a small volume of material weighing 0.012 grams.

The improper handling of ¹³⁷Cs gamma ray sources can lead to release ofthe radio-isotope and consequently radiation injuries. Cesium gamma-raysources that have been encased in metallic housings can be mixed-in withscrap metal on its way to smelters, resulting in production of steelcontaminated with radioactivity.

In oilfield operations isotopes can be lost into the well as a result ofthe breakage of the logging tool at the risk of being irretrievable.Such events can lead to the closure of the well or measures taken toensure that radioactive material cannot circulate or permeate out of thewell. Indeed, direct contamination and the risk to oilfield workers ofdangerous levels of exposure are not uncommon. Although comprehensivecontrol measures are in place, the risk associated with the use ofhighly radioactive isotopes during oilfield operations will always bepresent—unless a viable isotope-free option can be introduced.

As is the nature of radioactive materials, the half-life of the materialalso determines its useful lifetime. Although density logging tools arecalibrated to take into account the reduction in activity of an isotope,the useful life of the isotope is somewhat short-lived. A ¹³⁷Cs sourcewill be producing only half of its initial gamma-ray output after aperiod of 30 years. A consequence is that isotope-based sources need tobe replaced at intervals, and the older isotopes disposed of. Thedisposal requirements must take similar precautions to that of normalnuclear waste, such as that produced as a waste product at nuclear powerstations.

The typical regulatory limit for the amount of ¹³⁷Cs which may be usedduring a logging operation is a maximum of 1.3 Curie. During densitylogging operations, a certain number of photons per second are requiredto enter into the detectors to ensure a high enough statistic for thepurposes of data quality consistency and interpretation. As a result,density logging operations are normally performed such that the tool ismoved at a rate of 1,800 ft./hr. to ensure sufficient photons enter thedetectors at any particular depth to offer a data resolution acceptableto the client (typically a repeatability to 0.01 g/cc density). However,such a rate relates to open-hole logs, where the pad of the tool is indirect contact with the formation. In through-tubing or through-linerapplications, the rate of photons reaching the detectors would beseverely reduced by the attenuation of the photons by the productionliner, and the structure/materials immediately in the annulussurrounding the liner.

Such operations cannot currently be performed any faster, as the speedof logging relates to the acquisition speed that is proportional to theoutput of the gamma-source. For safety reasons, the amount of ¹³⁷Cs usedmay capped, with a resultant cap in the maximum amount of time availableto perform a log.

Various means have been published which attempt to mitigate this issueby using gamma-ray sources. However, none of the prior art teaches amethod of using x-rays to measure the variations in formation densityanticipated at the water-oil interface; detection/determination of thewater-oil interface through production liners; and techniques forcompensating for the production liner and liner-annular region whencomputing the formation density.

U.S. Pat. No. 8,481,919 to Teague teaches methods and means of creatingand controlling the required electrical power by serially stepping upthe DC reference and creating high potential field control surfaces, tocontrol either a bipolar or unipolar x-ray tube for the purposes ofreplacing chemical sources in reservoir logging. The reference alsoteaches moveable/manipulatable beam hardening filters and rotatinglight-house collimation on the source, the use of gaseous insulatorsincluding SF₆ as an electrical insulator in a downhole x-ray generator.However, it fails to disclose a method of using an x-ray technique tomeasure the variations in formation density anticipated at the water-oilinterface; detection/determination of the water-oil interface throughproduction liners; and techniques for compensating for the productionliner and liner-annular region when computing the formation density.

US2018/0188410 to Teague et al. teaches methods and means of using anelectronic x-ray device as a replacement for a chemical gamma-ray sourcewhen attempting to achieve a density computation to determine thedensity of a formation within an oil and gas well. The invention furtherteaches of a means of improving upon the accuracy of the measurement byusing the significantly higher output of an x-ray source (compared to1.5 Ci of ¹³⁷Cs) to increase the axial offset of a bulk-densitydetector, while maintaining the statistical requirements necessary toachieve 0.01 g/cc repeatability, thereby permitting a depth ofinvestigation that is outside of the ‘mud invaded’ zone of the formationwithin the oil & gas well. This method provides a framework for addingadditional data to the litho-density measurement, and provides a methodto remove uncertainty regarding mud-weight dependencies.

U.S. Pat. No. 7,675,029 to Teague et al. teaches and claims of the useof an x-ray device to create a two-dimensional image of a target objectin a borehole using backscattered radiation from an x-ray source bymeans of a collimated detector.

U.S. Pat. No. 7,292,942 to Ellis et al. discloses a method ofdetermining formation density in a cased hole environment using alogging tool having a gamma ray source; a long spacing detector; and ashort spacing detector that develops one or more cased hole calibrationrelationships utilizing differences between scattered gamma raysobserved by short spacing detectors and scattered gamma rays observed bylong spacing detectors to determine corrected formation density values;and then using the cased hole calibration relationships and scatteredgamma ray measurements obtained by the long spacing detector and theshort spacing detector to determine the formation density.

U.S. Pat. No. 6,182,013 to Malinverno et al. discloses methods forlocating an oil-water interface in a petroleum reservoir includingtaking resistivity and pressure measurements over time and interpretingthe measurements. The apparatus of the invention includes sensorspreferably arranged as distributed arrays. According to a first method,resistivity and pressure measurements are acquired simultaneously duringa fall-off test. Resistivity measurements are used to estimate theradius of the water flood front around the injector well based on knownlocal characteristics. The flood front radius and fall-off pressuremeasurements are used to estimate the mobility ratio.

U.S. Pat. No. 5,467,823 Babour et al. discloses a method and apparatusof monitoring subsurface formations containing at least one fluidreservoir and traversed by at least one well, by means of at least onesensor responsive to a parameter related to fluids, comprising the stepsof: lowering the sensor into the well to a depth level corresponding tothe reservoir; fixedly positioning said sensor at said depth whileisolating the section of the well where the sensor is located from therest of the well; and providing fluid communication between the sensorand the reservoir.

U.S. Pat. No. 5,642,051 to Babour et al. discloses a method and meansfor monitoring a fluid reservoir traversed by at least one wellcomprising the placing of at least one electrode in communication withthe surface and fixed in a permanent manner within the well.Hydraulically isolating the section of the well in which it is locatedfrom the rest of the well and providing electrical coupling between theelectrode and the reservoir. Subsequently, a current is passed throughthe reservoir; and an electrical parameter is measured, whereby acharacteristic representative of the reservoir can be deduced.

U.S. Pat. No. 5,767,680 to Torres-Verdin et al. discloses time-lapseDC/AC measurements made with an array of permanently deployed sensors inorder to detect and estimate the change in geometry and proximity of theoil-water interface as a result of production, and therefore as afunction of time. The estimation is carried out with a parametricinversion technique whereby the shape of the oil-water interface isassumed to take the form of a three-dimensional surface describable withonly a few unknown parameters. A nonlinear optimization technique isused to search for the unknown parameters such that the differencesbetween the measured data and the numerically simulated data areminimized in a least-squares fashion with concomitant hard boundphysical constraints on the unknowns. The proposed estimation procedureis robust in the presence of relatively high levels of noise and cantherefore be used to anticipate deleterious water breakthroughs, as wellas improve the efficiency with which the oil is produced from thereservoir.

U.S. Pat. No. 7,564,948 (Wraight, et al.) discloses an x-ray sourcebeing used as a replacement for a chemical source during density loggingalong with various means of arranging the apparatus and associatedpower-supply, also teaches of the means of filtering the primary beamfrom the x-ray source such that a filtered dual-peak spectrum can bedetected by a reference detector which is then used to directly control(feedback) the x-ray tube voltage and current for stability purposes.However, the references only discloses a compact x-ray device (bipolar)with a grid, a power supply, which is a Cockroft-Walton rolled up into acylinder (disposed between two Teflon cylinders) in order to save space,and the aforementioned filtered reference detector method.

SUMMARY

An x-ray based reservoir evaluation tool for measurement variations information density anticipated at the water-oil interface of a reservoiris provided, in which the tool includes at least: an internal lengthcomprising a sonde section, wherein said sonde section further comprisesan x-ray source; a plurality of radiation measuring detectors;sonde-dependent electronics; and a plurality of tool logic electronicsand PSUs.

A method of using an x-ray based reservoir evaluation tool for measuringvariations in formation density anticipated at the water-oil interfaceof a reservoir is also provided, the method including at least the stepsof: using x-rays to illuminate the formation surrounding the casedborehole; using a plurality of detectors to directly measure the densityof the formation; using detectors to directly measure the effects on themeasurement from tool stand-off or production liner attenuation; andcompensating for the production liner and liner-annular region whencomputing the saturated formation density within the productioninterval.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates an example x-ray-based reservoir evaluation toolbeing deployed into a borehole via wireline conveyance.

FIG. 2 illustrates another example of a physical layout of anx-ray-based reservoir evaluation tool.

FIG. 3 illustrates a still further example example of a physical layoutof an x-ray-based reservoir evaluation tool, and how the tool would beused to determine the location of the oil-water interface within areservoir through a production liner/casing.

BRIEF DESCRIPTION OF SEVERAL EXAMPLE EMBODIMENTS

The methods and means described herein for an electronic x-ray devicedescribe a means that measures variations in formation densityanticipated at the water-oil interface; detection/determination of thewater-oil interface through production liners; and techniques forcompensating for the production liner and liner-annular region whencomputing the saturated formation density within the productioninterval.

In the one embodiment, an x-ray based reservoir evaluation tool [101] isdeployed by wireline conveyance [103] into a cased borehole [102],wherein the tool is deployed into the reservoir to aid productionlogging. The tool is enclosed by a pressure housing which ensures thatwell fluids are maintained outside of the housing.

FIG. 2 is an example embodiment illustrating how the x-ray source [201]is located within a pressure-housing containing a padded section [211].The x-ray source is powered by one or more high-voltage generators[202]. A collimated detector [203, 204] is used to measure the offsetbetween the pad [211] and a production liner or tubing. In furtherembodiments, additional detectors [205, 206] are employed to measure theresponse from the production liner or tubing. The bulk density of thesaturated formation is measured by a long-offset detector [207]. Thedensity of the annular region surrounding the production liner ismeasured by a still further detector [208]. The pad-face of the tool ismaintained against the inner face of the production liner/tubing bymechanical methods [209]. The output from the detectors is processedwithin the downhole tool [210] electronics.

FIG. 3 is a further example embodiment illustrating how the x-ray source[201] produces a beam of x-rays [301] that penetrate through theproduction liner/casing [302] such that the saturated formation density,either containing oil [303] or water [305] are directly measured inorder to determine the location of the water-oil interface [304].Reference detectors [306] positioned to be optimized for measuringliner/casing/tubing and near-field annular material density are used tocompensate for variable material densities and attenuation profilesbetween the tool and the saturated formation.

For the reservoir to be a reservoir, the formation materials mustexhibit some porosity. The oil within the pores of the formationtypically has a lower density than the [salty] water located within thepores of the formation; consequently, the bulk-density of the formationmaterials itself is different on either side of the water-oil interface.These differences in bulk formation density, due to the saturation ofthe water and oil at difference depths, permits the possibility ofmeasuring the formation density using the Compton-scattered x-raysproduced when the x-ray source [201] illuminates the formation [303,305] while being conveyed through the production interval.

In one example embodiment, the near-field detectors [203, 204, 205, 206]are used to compensate for any near-field structural or materialattenuation effects that could adversely affect the quality of theformation density measurement. The result would be a density log,displayed as a function of what would clearly indicate the depth atwhich the water-oil interface is located. In a further embodiment, thex-ray source tube [201] is driven in a monopolar configuration. In astill further embodiment, the x-ray source tube [201] is driven in abipolar configuration. In a further embodiment still, the x-ray sourcetube [201] is monitored by a reference detector that measures thefluence of the source as a function of time, such that the primarydetector measurements may be compensated for. In yet another exampleembodiment, near-field liner/tubing offset detector [203, 204] is usedto determine whether the x-ray source beam [301] is passing through aperforation in the production liner/casing, such that this variance inthe computed long-space density can be noted as a function of depth.

In a further embodiment, near-field liner/tubing offset detector [203,204] is not required, if the outer diameter of the pad [6] is tailoredto match the inner diameter of the production liner. In a furtherembodiment, calibration techniques are employed prior the deployment ofthe tool such that the effect of various production liners andproduction tubing are parameterized to help inform the compensationscheme of data processing.

In a still further embodiment, the density log data is combinable withother measurements, such as neutron porosity, to improve the quality ofthe interpretation of the data and better determine the depth at whichthe water-oil interface exists. In a further embodiment still, existingopen-hole logs are used to establish a baseline profile as a function ofdepth for the formation density, such that sensitivity to the water-oilinterface is improved.

In a further embodiment, machine learning is used such that algorithmsare taught the indicators of the water-oil interface when comparing logsand log types, such that the process of determining the location of thewater-oil interface is then automated (i.e., requiring no initial humaninterpretation).

The foregoing specification is provided only for illustrative purposes,and is not intended to describe all possible aspects of the presentinvention. While the invention has herein been shown and described indetail with respect to several exemplary embodiments, those of ordinaryskill in the art will appreciate that minor changes to the description,and various other modifications, omissions and additions may also bemade without departing from the spirit or scope thereof.

1. An x-ray based reservoir evaluation tool for measurement variationsin formation density anticipated at the water-oil interface of areservoir, wherein said tool comprises: an internal length comprising asonde section, wherein said sonde section further comprises an x-raysource; a plurality of radiation measuring detectors; sonde-dependentelectronics; and a plurality of tool logic electronics and PSUs.
 2. Thetool of claim 1, further comprising a detector used to measure casingstandoff such that other detector responses are compensated for toolstand-off.
 3. The tool of claim 1, wherein said shield further comprisestungsten.
 4. The tool of claim 1, wherein the tool is configured so asto permit through-wiring.
 5. The tool of claim 1, wherein a referencedetector is used to monitor an azimuthal output of the x-ray source. 6.The tool in claim 1, wherein the tool is combinable with othermeasurement tools comprising one or more of neutron-porosity, naturalgamma and array induction tools.
 7. The tool in claim 1, wherein thetool is used to determine the position of the water-oil interfacethrough production liners or production casing.
 8. The tool in claim 1,wherein the tool uses additional axially-offset radiation detectors forcompensating for the production liner and liner-annular region effectswhen computing the formation density within the production interval. 9.The tool in claim 1, wherein the tool is integrated into alogging-while-drilling assembly.
 10. The tool in claim 1, wherein thetool is powered by mud-turbine generators.
 11. The tool in claim 1,wherein the tool is powered by batteries.
 12. The tool in claim 1,wherein the tool is combinable with other measurement tools comprisingone or more of neutron-porosity, natural gamma and array inductiontools.
 13. A method of using an x-ray based reservoir evaluation toolfor measuring variations in formation density anticipated at thewater-oil interface of a reservoir, wherein said method comprises: usingx-rays to illuminate the formation surrounding the cased borehole; usinga plurality of detectors to directly measure the density of theformation; using detectors to directly measure the effects on themeasurement from tool stand-off or production liner attenuation; andcompensating for the production liner and liner-annular region whencomputing the saturated formation density within the productioninterval.
 14. The method of claim 13, further comprising using adetector that is also used for measuring casing standoff so that otherdetector responses may be compensated for tool stand-off.
 15. The methodof claim 13, further comprising using a reference detector to monitorthe azimuthal output of an x-ray source.
 16. The method of claim 13,further comprising combining other measurement methods comprising one ormore of neutron-porosity, natural gamma and array induction tools. 17.The method of claim 13, further comprising using the tool to determinethe position of the water-oil interface through production liners orproduction casing.
 18. The method of claim 13, further comprising usingadditional axially-offset radiation detectors to compensate for theproduction liner and liner-annular region effects when computing theformation density within the production interval.
 19. The method ofclaim 13, further comprising integrating the tool into alogging-while-drilling assembly.
 20. The method of claim 13, furthercomprising powering the tool using mud-turbine generators.
 21. Themethod of claim 13, further comprising powering the tool usingbatteries.